Nondestructive testing of ferromagnetic objects by use of eddy currents is well known. This type of testing establishes eddy currents in a workpiece being tested which in turn establishes flux fields. A flaw or defect in the workpiece causes discontinuities in eddy current flow and result in corresponding discontinuities in the flux field. When a detection coil is moved along the workpiece, flux field variations are sensed as it passes over a flaw.
Multiple probe eddy current flaw detection devices are known for inspecting the entire circumference of an object, such as a tube or pipe, for the presence of defects. One such detection device includes a plurality of pickup arms mounted in spaced relationship around a workpiece travel path. Each of the pickup arms carries a set of sensing coils. Each pickup arm is mounted to pivot on an axis transverse to the direction of the travel path so that the sensing coil sets may be moved to closely surround a workpiece as it travels along the path.
Such a device is disclosed in U.S. Pat. No. 4,101,832 to Baker et al which has been useful to detect the presence of defects in an entire surface of a workpiece such as a pipe. In the Baker patent, each coil set is pre-formed for the inspection of workpieces, all of which have essentially the same uniform cross-sectional configuration from end to end. When utilizing the Baker et al device to inspect pipe, the detector sets are configured for the inspection of pipe of a given predetermined diameter. To change, for example, from inspection of six inch pipe to ten inch diameter pipe using the Baker et al device, it is necessary to install new detector sets configured to a larger diameter.
Another defect detecting apparatus is disclosed in U.S. Pat. No. 3,593,120 to Mandula, Jr. This apparatus includes a test probe that moves rotationally about the object being tested. The apparatus comprises a yoke having a pair of arms flexibly connected to and spanned by a chain of rolling members supporting a test probe. The rolling chain travels on the surface of the article so that the single probe is spring biased into testing contiguity not only with flat surfaces but also with rounded corners and curved surfaces of an article being tested such as a billet. The probe and the article being tested move in relative rotational and linear motion so that the probe spirally scans the inspected surface.
The Mandula device, does not provide the capability of multiprobe testing of the type that circumferentially surrounds a workpiece. Moreover, the rotational and linear relative motion is obviously more difficult to achieve than in simple linear motion as used by Baker et al.